The investigation of surface composition of nitrogen-doped niobium for superconducting RF cavities.

Systematic analysis of the surface morphology, crystalline phase, chemical composition and elemental distribution along depth for nitrogen-doped niobium was carried out through different ways of characterization, including SEM, AFM, GIXRD, RBS and layer-by-layer XPS analysis. The results showed that, after nitrogen doping, the surface was covered by densely distributed trigonal precipitates with average crystallite size of 32 ± 8 nm, in line with the calculation result (29.9 nm) of nitrogen-enriched β-Nb2N from GIXRD, demonstrating the phase composion of trigonal precipitates. The depth analysis through RBS and XPS indicated that β-Nb2N was dominant in the topmost 9.7 nm and extended to depth of 575 nm with gradually decreased content. In addition, the successive change of naturally oxidized states of niobium after nitrogen doping along depth was revealed along depth. It was interesting to find that oxygen diffusion depth could be moderately enhanced by nitridation process. These results established the near-surface phase composition of nitrided niobium, which was of great significance for evaluating the effect of nitrogen doping and further understanding the Q improvement of the SRF cavities.